The invention relates to aircraft towbars, and more particularly to towbars designed to prevent damage to the nosewheel of a towed aircraft due to overly sharp turning of the towing vehicle.
In the nosewheel landing gear assembly of an airplane, the turning movement of the nosewheel and the nosewheel fork in either direction for steering purposes is limited and a substantial turning force applied to the wheel or wheel fork tending to turn the fork beyond the turning limits will result in damage to the nosewheel assembly. Such a force can occur when a towing vehicle, connected to the nosewheel assembly by a towbar aligned with the nosewheel, turns too sharply. It can also occur in backing the aircraft, even though the vehicle may be moving straight backward.
Aircraft towbars having various means for preventing the above described damage to the nosewheel assembly of a towed aircraft have been suggested. The most common type of such towbar is shown in U.S. Pat. No. 2,468,669. This type towbar includes a pair of shear pins along the length of the towbar, retaining forward and rearward sections of the towbar together. When the towbar is subjected to a certain magnitude of bending stress caused by an attempt to turn the aircraft nosewheel too sharply, at least one of the shear pins fails so that damage to the nosewheel assembly does not occur. U.S. Pat. No. 2,773,703 shows such a towbar wherein one of the two shear pins acts as a pivot pin when the second, weaker shear pin fails.
Another type towbar designed to prevent aircraft nosewheel damage and including a pivot pin is shown in U.S. Pat. No. 3,004,773. However, no shear pins are included in this design. Instead, at a position spaced from the pivot point of the towbar, a pair of slidably mounted rollers biased downwardly by a spring and connected to the forward portion of the towbar are normally engaged in a notch or detent of a generally horizontal plate extending from the rearward portion of the towbar. When the rollers are in their normal position in the detent, the forward and rearward sections of the towbar are aligned in colinear relationship. Only if a substantial bending stress is placed on the towbar do the rollers swing out of the detent, allowing the forward section of the towbar to swing with respect to the rearward section about the pivot point. This occurs if the towing vehicle turns too sharply while towing the aircraft. The towbar is not reset until movement of the towing vehicle straightens out the two sections of the towbar to bring the rollers into the detent.
While the safety towbars discussed are generally effective to prevent damage to aircraft nosewheel assemblies, they have certain shortcomings. A shear pin type towbar must of course be reset by installation of new shear pins after removal of the old shear pin fragments. To accomplish this, the two sections of the towbar must be properly aligned, and the procedure can be somewhat troublesome and time consuming. In the other type towbar discussed above, where the pivoting towbar is reset only by proper movement of the towing vehicle, a temporary loss of control can occur. For example, if when the two sections of the towbar are pivoted, it is desirable to move the aircraft backward, a jackknifing of the pivoted towbar assembly can very easily result, even after the aircraft nosewheel has moved back toward center. Steering control in the forward direction is also lacking until the towbar has been reset. In addition, this type safety towbar may be inconsistent in the bending stress values at which the pivoting feature is deployed. Such deployment is subject to variations due to wear and changing atmospheric conditions which affect friction.